3B &C). Open in a separate window Figure 3 UV exposure inhibits Tfh generation. germinal center formation. Our findings demonstrate a novel function for mast cells, suppression of Tfh production, GC formation and antibody production in vivo. Introduction T-dependent antibody responses depend on the generation of germinal centers (GC), which are specialized structures present in B cell follicles in secondary lymphoid tissues. The B cells JNJ-26481585 (Quisinostat) found in these structures have a high rate of proliferation and are identified by peanut agglutinin (PNA) binding and BCL-6 expression (1, 2). Within the GC class switching, recombination, somatic hypermutation, and selection of high affinity B cells occurs (3, 4). For years it was recognized that CD4+ T helper cell function was critical for GC formation by providing help to antigen-specific B cells and promoting the differentiation of plasma and memory cells. More recently, a specialized subset of CD4+ T cells, called T follicular helper (Tfh) cells was identified that provide help for GC and antibody formation (5). Tfh are characterized by expression of co-stimulatory molecules such as ICOS, CD40L, CTLA-4, PD-1 and JNJ-26481585 (Quisinostat) BTLA, and by the intense JNJ-26481585 (Quisinostat) and sustained expression of CXCR-5 (6-8). The transcription factor that mediates the development of Tfh is BCL-6, whereas BLIMP-1 antagonizes the activity of BCL-6 and inhibits Tfh development (9-11). Autocrine production of IL-21 is fundamental for Tfh activation and consequently GC formation and antibody production (12, 13). UV radiation is one of the most common environmental factors affecting human health. The UV wavelengths present in sunlight contribute significantly to the development of skin cancer (14), the most prevalent type of cancer found in the United States (15). Besides its carcinogenic effect, it is well known that exposure to UV radiation is immune suppressive, as demonstrated by the inhibition of cell-mediated immune reactions such as contact and delayed type hypersensitivity (16, 17). A less well-recognized result of total body UV exposure is the suppression of T-dependent, but not T-independent antibody formation (18-21). Although IL-10 producing T cells have been implicated in this process, the exact mechanism(s) leading to UV-induced suppression of antibody formation are not well defined. Following UV exposure, several cell populations are implicated in the process leading to immune suppression, including keratinocytes (22), macrophages (23), Langerhans cells (24), NKT cells (25), IL-10 secreting CD4+CD25+ T regulatory cells (26) and mast cells (27). In addition to their well-characterized role in type I hypersensitivity, mast cells have the potential to diminish inflammation and suppress immune responses (28). One of the first examples of mast cells playing a role in regulating adaptive immunity was the suppression of contact and delayed type hypersensitivity following UV exposure (27). Moreover, mast cell migration from UV-irradiated skin to the draining lymph node represents a mechanism by which an immune suppressive signal is transmitted from the skin to the JNJ-26481585 (Quisinostat) immune system (29). In addition, it has been shown that IL-10 produced by mast cells has the ability to limit inflammation in the skin (30) and it has been suggested that mast cell-derived IL-10 is essential for tolerance induction following UV exposure (31). Here RHEB we examined the role of mast cells in the suppression of antibody formation. Exposing mice to UV radiation suppresses GC formation, antibody formation,.